1. Field of the Invention
The present invention relates to a sealed battery manufactured by sealing power generating elements, such as an electrode group and an electrolyte solution, in a battery case, and to a method for manufacturing the same.
2. Description of Related Art
With the advance of electrical equipment of various portable types, the development of batteries serving as the driving source becomes of increasing importance. Of all the types of batteries, small-sized rechargeable batteries, such as nickel-metal hydride rechargeable batteries and lithium ion rechargeable batteries, have been used as driving sources in hybrid vehicles in addition to cellar phones, notebook-sized personal computers, video cameras, etc., and the demand has been on the increase.
Such batteries are classified broadly into a cylindrical type and a prismatic type. The cylindrical battery has high energy density per unit volume because of the structure in which a spiral electrode group fabricated by winding strip-like positive electrode plate and negative electrode plate with a separator being interposed therebetween is accommodated in a battery case. On the other hand, as shown in FIG. 7, the prismatic battery generally has a structure in which an electrode group 104 fabricated by sequentially depositing a positive electrode plate 101 and a negative electrode plate 102 with a separator 103 being interposed therebetween is accommodated in a battery case 105. Comparison in terms of the structures reveals that the cylindrical battery excels in productivity and costs less. However, comparison with a comparative example of a battery of the invention shown in FIG. 5B reveals that, when a battery pack 17B is formed by accommodating a plurality of cylindrical batteries B in a pack case 18, a dead space is increased and space efficiency is thereby deteriorated; what is worse, the stability of the batteries within the housing space is poor. Hence, the cylindrical battery is not suitable for use in reducing the size and the thickness of electrical equipment.
On the contrary, the prismatic battery is advantageous in that a plurality of batteries can be accommodated in the housing space at high space efficiency and in a stable manner. However, since the electrode group 104 is of a laminated structure as described above, when the number of electrode plates 101 and 102 is increased with the aim of increasing the battery capacity, the production costs are increased because of the need to increase the number of connection lines 106 (see FIG. 7) and the like. Besides, the prismatic battery has a drawback that it is not readily sealed in comparison with the cylindrical battery, and thus, it becomes less reliable.
In the case of the prismatic battery, a popular method adopted as the sealing method is to weld a junction portion between an opening-sealing plate and a case opening through laser welding. This method, however, not only increases the manufacturing costs, but also makes it difficult to manage the laser condition in stabilizing the welding state, and therefore, cannot be recommended as a highly reliable method. Further, in the case of the prismatic battery as shown in FIG. 7, Japanese Patent Laid-Open Publication No. Sho. 63-221551, for example, discloses an opening sealing method that seals an opening with an opening-sealing member 110 through caulking as with the cylindrical battery. According to this method, however, it is difficult to form an annular groove, needed to seal the opening with the sealing member 110, on the side surface of the battery case. Further, when the opening end portion of the prismatic tubular case is caulked, distortion occurs often at each corner portion. Hence, this method has a problem that the air-tightness is poor in comparison with the case of caulking the cylindrical battery.
This problem would be eliminated by a method of providing, as shown in FIG. 9, a cylindrical opening head 211 to a battery case 205 having a prismatic tubular barrel portion 212, and then caulking the opening head 211. As an example of such a caulking method, a manufacturing method of a battery employing the caulking method disclosed in, for example, Japanese Patent Laid-Open Publication No. Sho. 58-112259 will now be described.
As shown in FIG. 8, after an electrode group (not shown) is accommodated in the battery case 205, the bottom portion of the battery case 205 is inserted into a holder 207. Then, an opening of the battery case 205 is fixed by an upper-portion fixing device 208 while a pressing force is kept applied to the battery case 205 in the axial direction from the bottom portion side as indicated by an arrow. Subsequently, the battery case 205 is rotated by rotating the upper-portion fixing device 208 at a certain number of rotations. Under these conditions, a groove-forming roller 209 is press-adhered to the side surface of the battery case 205 while the battery case 205 is kept pressed in the axial direction from the bottom portion side, whereby an annular groove 205a is formed in the vicinity of the opening. While having an annular supporting portion 205b that bulges inward due to the annular groove 205a support an opening-sealing member 210 (FIG. 9), the opening of the battery case 205 is bent inward and the opening-sealing member 210 is thereby fixed between the opening end portion and the annular supporting portion 205b through caulking. Consequently, the battery is sealed airtight.
However, when the conventional annular groove forming method as described above is applied to the case of manufacturing a battery having the prismatic tubular barrel portion 212 and the cylindrical opening head 211 as shown in FIG. 9, the pressing force applied to the battery case 205 in the axial direction from the bottom portion side makes it difficult to smoothly supply the material used to form the annular groove 205a from the barrel portion 212 side. Hence, when the annular groove 205a is formed at the boundary portion between the opening head 211 and the barrel portion 212, the boundary portion is extended locally by the groove-forming roller 209 while the material is hardly supplied. This reduces the thickness of the locally extended portion, and causes a problem, such as deformation and breaking.
Hence, when the annular groove 205a is formed by the conventional method, as shown in FIG. 6B, the annular groove 205a has to be formed while securing a cylindrical portion 205c of an adequate size below the groove 205a in order to ensure a supply of the material. However, a useless space is left at the inner side of the cylindrical portion 205c, which raises a problem that the volume energy density is reduced.